Systems and methods for reducing intracranial pressure

ABSTRACT

In one embodiment, the invention provides a device for decreasing intracranial or intraocular pressures. The device comprises a housing having an inlet opening and an outlet opening that is adapted to be interfaced with a person&#39;s airway. The device further includes a valve system that is operable to regulate respiratory gas flows through the housing and into the person&#39;s lungs during spontaneous or artificial inspiration. The valve system assists in lowering intrathoracic pressures during each inspiration to repetitively lower pressures in the venous blood vessels that transport blood out of the head to thereby reduce intracranial or intraocular pressures.

BACKGROUND OF THE INVENTION

This invention relates generally to the field of intracranial and intraocular pressures. More specifically, the invention relates to devices and methods for decreasing intracranial and intraocular pressures, such as those resulting from a traumatic head injury.

Head trauma is generally regarded as the leading cause of morbidity and mortality in the United States for children and young adults. Head trauma often results in swelling of the brain. Because the skull cannot expand, the increased pressures within the brain can lead to death or serious brain injury. While a number of therapies have been evaluated in order to reduce brain selling, including use of hyperventilation and steroids, an effective way to treat intracranial pressures remains an important medical challenge.

BRIEF SUMMARY OF THE INVENTION

In one embodiment, the invention provides a device for decreasing intracranial or intraocular pressures. The device comprises a housing having an inlet opening and an outlet opening that is adapted to be interfaced with a person's airway. The device further includes a valve system that is operable to regulate respiratory gas flows through the housing and into the person's lungs during spontaneous or artificial inspiration. The valve system assists in lowering intrathoracic pressures during inspiration to continuously or intermittently lower pressures in the venous blood vessels that transport blood out of the head to thereby reduce intracranial or intraocular pressures.

In one aspect, the valve system is configured to open to permit respiratory gasses to freely flow to the person's lungs when the negative intrathoracic pressure reaches a pressure in the range from about −2 cmH2O to about −20 cmH2O in order to reduce intracranial or intraocular pressures. In this way, the negative intrathoracic pressure is lowered until a threshold pressure is reached, at which time the valve opens. The cycle may be repeated continuously or periodically to repetitively lower intrathoracic pressures.

The device may also include means for causing the person to artificially inspire through the valve system. For example, the device may utilize an electrode, an iron lung cuirass device, a chest lifting device, a ventilator or the like.

The device may further include a mechanism for varying the level of impedance of the valve system. This may be used in combination with at least one physiological sensor that is configured to monitor at least one physiological parameter of the person. In this way, the mechanism for varying the level of impedance may be configured to receive signals from the sensor and to vary the level of impedance of the valve system based on the signals. Examples of sensors that may be used include those that measure respiratory rate, intrathoracic pressure, intratracheal pressure, blood pressure, heart rate, end tidal CO2, oxygen level, and intracranial pressure.

In one aspect, a coupling mechanism may be used to couple the valve system to the person's airway. Examples of coupling mechanisms include a mouthpiece, an endotracheal tube, and a face mask.

A wide variety of valve systems may be used to repetitively decrease the person's intrathoracic pressure. For example, valve systems that may be used include those having spring-biased devices, those having automated, electronic or mechanical systems to occlude and open a valve lumen, duck bill valves, ball valves, other pressure sensitive valve systems capable of opening a closing when subjected to low pressure differentials triggered either by spontaneous breathing and/or external systems to manipulate intrathoracic pressures (such as ventilators, phrenic nerve stimulators, iron lungs, and the like).

In another embodiment, the invention provides a method for decreasing intracranial or intraocular pressures. According to the method, a valve system is coupled to a person's airway and is configured to at least periodically reduce or prevent respiratory gases from flowing to the person's lungs. With the valve system coupled to the airway, the person's negative intrathoracic pressure is repetitively decreased to in turn repetitively lower pressures in the venous blood vessels that transport blood out of the head. In so doing, intracranial and intraocular pressures are reduced.

The person's negative intrathoracic pressure may be repetitively decreased as the person repeatedly inspires through the valve system. This may be done by the person's own efforts (referred to as spontaneous breathing), or by artificially causing the person to repeatedly inspire through the valve system. For example, the person may be caused to artificially inspire by repeatedly stimulating the phrenic nerve, by manipulating the chest with an iron lung cuirass device, by generating negative pressures within the thorax using a ventilator, by applying a high frequency ventilator that supplies oscillations at a rate of about 200 to about 2000 per minute, or the like.

In another aspect, the level of impedance of the valve system may be fixed or variable. If variable, at least one physiological parameters of the person may be measured, and the impedance level may be varied based on the measured parameters.

To couple the valve system to the airway, a variety of techniques may be used, such as by using a mouthpiece, an endotracheal tube, a face mask or the like. Further, the respiratory gases may be prevented from entering the lungs through the valve system until a negative intrathoracic pressure in the range from about 0 cmH2O to about −25 cmH2O is achieved, at which time the valve system permits respiratory gases to flow to the lungs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating one method for reducing intracranial and intraocular pressures according to the invention.

FIG. 2 is a perspective view of one embodiment of a facial mask and a valve system that may be used to reduce intracranial and intraocular pressures according to the invention.

FIG. 3 is a perspective view of the valve system of FIG. 2.

FIG. 4 is a cross sectional side view of the valve system of FIG. 3.

FIG. 5 is an exploded view of the valve system of FIG. 3.

FIG. 6 is a schematic diagram of a system for reducing intracranial and intraocular pressures according to the invention.

FIG. 7 is a series of graphs illustrating the lowering of intracranial pressures in an animal study.

FIG. 8 is a series of graphs illustrating the lowering of intracranial pressures in another animal study.

DETAILED DESCRIPTION OF THE INVENTION

In a broad sense, the invention provides devices and techniques for lowering intracranial and intraocular pressures. Such devices and techniques may be particularly helpful with patients who have suffered a traumatic brain injury. One way to lower such pressures is by using a valve system that is coupled to a person's airway and that is used to lower intrathoracic pressures. In so doing, the valve systems may be used to accelerate the removal of venous blood from the brain, thereby decreasing intracranial and intraocular pressures.

More specifically, intracranial pressures are regulated by the amount the cerebral perfusion pressure, which is determined by the arterial blood pressure to the head, the pressures within the skull, and the pressures within the venous system that drains blood flow from the brain. The devices and methods of the invention may be used to enhance the egress of venous blood out of the brain, thereby lowering intracranial pressures. To do so, the devices and methods may be used to augment the intrathoracic vacuum effect each time a patient inhales (or in the case of a non-breathing patient, each time the pressure within the chest is manipulated to fall below atmospheric pressure), thereby lowering the pressures in the thorax and in the venous blood vessels that transport blood out of the brain. The vacuum effect is transduced back into the brain, and as a result, intracranial pressures are lowered with each inspiratory effort. This in turn causes more venous blood to flow out of the head than would otherwise be possible, resulting in lower intracranial pressures and lower intraocular pressures.

To prevent or impede respiratory gases from flowing to the lungs, a variety of impeding or preventing mechanisms may be used, including those described in U.S. Pat. Nos. 5,551,420; 5,692,498; 6,062,219; 5,730,122; 6,155,257; and 6,224,562, and in U.S. patent application Ser. No. 10/224,263, filed on Aug. 19, 2002 (“Systems and Methods for Enhancing Blood Circulation”, U.S. patent application Ser. No. 10/401,493, filed Mar. 28, 2003 (“Diabetes Treatment Systems and Methods”, U.S. patent application Ser. No. 09/966,945, (“Face Mask Ventillation/Perfusion Systems & Method”), filed Sep. 28, 2001 and U.S. patent application Ser. No. 09/967,029, (“Systems and Methods to Facilitate the Delivery of Drugs”), filed Sep. 28, 2001, the complete disclosures of which are herein incorporated by reference. The valve systems may be configured to completely prevent or provide resistance to the inflow of respiratory gases into the patient while the patient inspires. For valve systems that completely prevent the flow of respiratory gases, such valves may be configured as pressure responsive valves that open after a threshold negative intrathoracic pressure has been reached.

For example, the resistance to the inflow of respiratory gases may be set between about 0 cm H2O and about −25 cm H2O and may be variable or fixed. More preferably, the valve system may be configured to open when the negative intrathoracic pressure is in the range from about −2 cmH2O to about −20 cmH2O.

Although not intended to be limiting, specific kinds of impedance valves that may be used to reduce intracranial and intraocular pressures include those having spring-biased devices, automated/electronic and mechanical means to occlude and open a valve lumen, duck bill valves, ball valves, and other pressure sensitive valve systems capable of opening and closing when subjected to low pressure differentials triggered either by spontaneous breathing and/or external means to manipulate intrathoracic pressure (such as ventilators, phrenic nerve stimulators, an iron lung, and the like).

In the past, such threshold valve systems have been used to increase the venous preload on the heart and to increase cardiac output, stroke volume and blood pressure because of the augmented effects of the intrathoracic vacuum on the subsequent cardiac contraction. In contrast, the techniques of the invention function by facilitating the removal of blood from the venous side of the brain. Although there may be an increase in blood flow out of the heart to the vital organs (including to the brain) when using such valve systems, the effect of the valve systems on lowering of intracranial pressures was quite unexpected because of the known increase in blood flow to the brain. Remarkably, however, the reduction of venous blood pressures from the brain remains substantial when using the valve systems. Thus, despite the increase in blood flow to the brain, the net effect of the valve system is a decrease in intracranial pressures.

With the valve system coupled to the person's airway, the negative intrathoracic pressure may be enhanced by inspiring through the valve system. If the person is spontaneously breathing, the person may simply breath through the valve system. If the person is not breathing, artificial inspiration may be induced using a variety of techniques. For example, at least some of the respiratory muscles, and particularly the inspiratory muscles, may be stimulated, to contract in a repeating manner in order to cause the person to inspire through the valve system, thereby increasing the magnitude and prolonging the duration of negative intrathoracic pressure, i.e., respiratory muscle stimulation increases the duration and degree that the intrathoracic pressure is below or negative with respect to the pressure in the peripheral venous vasculature. Upon contraction of the respiratory muscles, the patient will typically “gasp”.

Among the respiratory muscles that may be stimulated to contract are the diaphragm, the chest wall muscles, including the intercostal muscles and the abdominal muscles. Specific chest wall muscles that may be stimulated to contract include those that elevate the upper ribs, including the scaleni and sternocleidomastoid muscles, those that act to fix the shoulder girdle, including the trapezii, rhomboidei, and levatores angulorum scapulorum muscles, and those that act to elevate the ribs, including the serrati antici majores, and the pectorales majores and minores as described generally in Leslie A. Geddes, “Electroventilation—A Missed Opportunity?”, Biomedical Instrumentation & Technology, July/August 1998, pp. 401–414, the complete disclosure of which is herein incorporated by reference. Of the respiratory muscles, the two hemidiaphragms and intercostal muscles appear to be the greatest contributors to inspiration and expiration. The respiratory muscles may be stimulated to contract in a variety of ways. For example, the diaphragm may be stimulated to contract by supplying electrical current or a magnetic field to various nerves or muscle bundles which when stimulated cause the diaphragm to contract. Similar techniques may be used to stimulate the chest wall muscles to contract. A variety of pulse trains, pulse widths, pulse frequencies and pulse waveforms may be used for stimulation. Further, the electrode location and timing of pulse delivery may be varied. In one particular aspect, an electrical current gradient or a magnetic field is provided to directly or indirectly stimulate the phrenic nerve.

To electrically stimulate the inspiratory motor nerves, electrodes are preferably placed on the lateral surface of the neck over the point where the phrenic nerve, on the chest surface just lateral to the lower sternum to deliver current to the phrenic nerves just as they enter the diaphragm, on the upper chest just anterior to the axillae to stimulate the thoracic nerves, in the oral pharyngeal region of the throat, or on the larynx itself. However, it will be appreciated that other electrode sites may be employed. For example, in one embodiment the respiratory muscles are stimulated by a transcutaneous electrical impulse delivered along the lower antero-lat margin of the rib cage. In one embodiment, inspiration is induced by stimulating inspiratory muscles using one or more electrodes attached to an endotracheal tube or pharyngeal tube. To stimulate the diaphragm, the phrenic nerve may be stimulated in the neck region near C3–C7, such as between C3, C4 or C5, or where the phrenic nerves enter the diaphragm. Alternative techniques for stimulating diaphragmatic contraction include magnetic field stimulation of the diaphragm or the phrenic nerve. Magnetic field stimulation may also be employed to stimulate the chest wall muscles. Electrical field stimulation of the diaphragm or the chest wall muscles may be accomplished by placing one or more electrodes on the skin, preferably in the vicinity of the neck or the lower rib cage (although other locations may be employed) and then providing an electrical voltage gradient between electrodes that induces transcutaneous current flow to stimulate the respiratory muscles to contract. Still further, subcutaneous electrodes may also be used to stimulate respiratory muscle contraction. Other techniques are described in U.S. Pat. No. 6,463,327, the complete disclosure of which is herein incorporated by reference.

The valve systems may have a fixed actuating pressure or may be variable so that once a desired negative intrathoracic pressure is reached, the resistance to flow may be lessened. Further, the valves of the invention may be configured to be variable, either manually or automatically. The extent to which the resistance to flow is varied may be based on physiological parameters measured by one or more sensors that are associated with the person being treated. As such, the resistance to flow may be varied so that the person's physiological parameters are brought within an acceptable range. If an automated system is used, such sensors may be coupled to a controller which is employed to control one or more mechanisms that vary the resistance or actuating pressure of the inflow valve as generally described in the references that have been incorporated by reference.

Hence, the valve systems of the invention may also incorporate or be associated with sensors that are used to detect changes in intrathoracic pressures or other physiological parameters. In one aspect, the sensors may be configured to wirelessly transmit their measured signals to a remote receiver that is in communication with a controller. In turn the controller may use the measured signals to vary operation of the valve systems described or incorporated by reference herein. For example, sensors may be used to sense blood pressure, pressures within the heart, intrathoracic pressures, positive end expiratory pressure, respiratory rate, intracranial pressures, intraocular pressures, respiratory flow, oxygen delivery, temperature, blood pH, end tidal CO2, tissue CO2, blood oxygen, cardiac output or the like. Signals from these sensors may be wirelessly transmitted to a receiver. This information may then be used by a controller to control the actuating pressure or the resistance of an inflow valve as described in the references incorporated herein by reference.

FIG. 1 is flow diagram illustrating one method for reducing intracranial or intraocular pressures. As shown in step 10, the process proceeds by coupling a valve system to the person's airway. Any kind of coupling mechanism may be used, such as by a mouthpiece, an endotracheal tube, a face mask, or the like. Further, any of the valve systems described or incorporated herein by reference may be used. In step 20, the person's negative intrathoracic pressure is repetitively decreased (either artificially or by spontaneous breathing). Examples of techniques to artificially reduce the negative intrathoracic pressure include use of an iron lung cuirass device, a ventilator that is capable of generating negative pressures, a ventilator that is capable of providing high frequency oscillations at a rate of about 200 to about 2000 per minute, a phrenic nerve stimulator, or the like. As the person's negative intrathoracic pressure is repeatedly decreased while the valve system is coupled to the airway, the pressures in the venous vessels that transport blood out of the head are also lowered. In so doing, intracranial and intraocular pressures are reduced.

As shown in step 30, various physiological parameters of the person may optionally be measured. Examples of such parameters include respiratory rate, intrathoracic pressure, intertracheal pressure, intracranial pressure, intraocular pressure, blood pressure, heart rate, end tidal CO₂, oxygen saturation, and the like. Further, as shown in step 40, the valve system's actuating threshold level may optionally be varied based on the measured physiological parameters. This may be done to maximize the amount of blood drawn out of the brain or simply to monitor the patient's condition to insure that the patient remains stable.

FIG. 2 illustrates one embodiment of a facial mask 100 to which is coupled a valve system 200. Mask 100 is configured to be secured to a patient's face so as to cover the mouth and nose. Mask 100 and valve system 200 are examples of one type of equipment that may be used to lower intrathoracic pressures and thereby lower intracranial and intraocular pressures. However, it will be appreciated that other valve systems and other coupling arrangements may be used including, for example, those previously referenced. As such the invention is not intended to be limited to the specific valve system and mask described below.

Referring also to FIGS. 3–5, valve system 200 will be described in greater detail. Valve system 200 includes a valve housing 202 with a socket 204 into which a ball 206 of a ventilation tube 208 is received. In this way, ventilation tube 208 may rotate about a horizontal axis and pivot relative to a vertical axis. A respiratory source, such as a ventilation bag, may be coupled to tube 208 to assist in ventilation. Disposed in ventilation tube 208 is a filter 210 that is spaced above a duck bill valve 212. A diaphragm holder 214 that holds a diaphragm 216 is held within housing 202. Valve system 200 further includes a patient port 218 that is held in place by a second housing 220. Housing 220 conveniently includes tabs 222 to facilitate coupling of valve system 200 with facial mask 100. Also held within housing 220 is a check valve 224 that comprises a spring 224 a, a ring member 224 b, and an o-ring 224 c. Spring 224 a biases ring member 224 b against patient port 218. Patient port 218 includes bypass openings 226 that are covered by o-ring 224 c of check valve 224 until the pressure in patient port 218 reaches a threshold negative pressure to cause spring 224 a to compress.

When the patient is actively ventilated, respiratory gases are forced through ventilation tube 208. The gases flow through filter 210, through duck bill valve 212, and forces up diaphragm 216 to permit the gases to exit through port 218. Hence, at any time the patient may be ventilated simply by forcing the respiratory gases through tube 208.

During the exhalation phase of a breathing cycle, expired gases flow through port 218 and lift up diaphragm 216. The gases then flow through a passage 227 in ventilation tube 208 where they exit the system through openings 229 (see FIG. 3).

During the inhalation phase of a breathing cycle, valve system 200 prevents respiratory gases from flowing into the lungs until a threshold negative intrathoracic pressure level is exceeded. When this pressure level is exceeded, check valve 224 is pulled downward as springs 224 a are compressed to permit respiratory gases to flow through openings 226 and to the patient's lungs by initially passing through tube 208 and duck bill valve 212. Valve 224 may be set to open when the negative intrathoracic pressure is in the range from about 0 cm H2O to about −25 cm H2O, and more preferably from about −2 cm H2O to about −20 cm H2O. Hence, the magnitude and duration of negative intrathoracic pressure may be enhanced during patient inhalation by use of valve system 200. Once the intrathoracic pressure falls below the threshold, recoil spring 224 a again close check valve 224. In this way, pressure within the venous blood vessels that transport blood out of the brain are also lowered. In so doing, more blood is drawn out of the brain to reduce intracranial and intraocular pressures.

Any of the valve systems described herein may be incorporated into a treatment system 300 as illustrated in FIG. 6. System 300 may conveniently include facial mask 100 and valve system 200, although any of the valve systems or interfacing mechanisms described herein or the like may be used. Valve system 200 may conveniently be coupled to a controller 310. In turn, controller 310 may be used to control the impedance level of valve system 200 in a manner similar to any of the embodiments described or incorporated herein. The level of impedance may be varied based on measurements of physiological parameters, or using a programmed schedule of changes. System 300 may include a wide variety of sensors and/or measuring devices to measure any of the physiological parameters described herein. These sensors or measuring devices may be integrated within or coupled to valve system 200 or facial mask, or may be separate.

For example, valve system 200 may include a pressure transducer for taking pressure measurements (such as intrathoracic pressures, intracranial pressures, intraocular pressures), a flow rate measuring device for measuring the flow rate of air into or out of the lungs, or a CO2 sensor for measuring expired CO2.

Examples of other sensors or measuring devices include a heart rate sensor 330, a blood pressure sensor 340, and a temperature sensor 350. These sensors may also be coupled to controller 310 so that measurements may be recorded. Further, it will be appreciated that other types of measuring devices may be used to measure various physiological parameters, such as oxygen saturation and/or blood levels of O2, blood lactate, blood pH, tissue lactate, tissue pH, blood pressure, pressures within the heart, intrathoracic pressures, positive end expiratory pressure, respiratory rate, intracranial pressures, intraocular pressures, respiratory flow, oxygen delivery, temperature, end tidal CO2, tissue CO2, cardiac output or the like.

In some cases, controller 310 may be used to control valve system 200, to control any sensors or measuring devices, to record measurements, and to perform any comparisons. Alternatively, a set of computers and/or controllers may be used in combination to perform such tasks. This equipment may have appropriate processors, display screens, input and output devices, entry devices, memory or databases, software, and the like needed to operate system 300.

A variety of devices may also be coupled to controller 310 to cause the person to artificially inspire. For example, such devices may comprise a ventilator 360, an iron lung cuirass device 370 or a phrenic nerve stimulator 380. Ventilator 360 may be configured to create a negative intrathoracic pressure within the person, or may be a high frequency ventilator capable of generating oscillations at about 200 to about 2000 per minute.

EXAMPLE

The following is a non-limiting example illustrating how intracranial pressures may be lowered according to the invention. In this example, 30 kg pigs were anesthetized with propofol. Using a micromannometer-tipped electronic Millar catheter inserted below the dura, intracranial pressures were measured continuously in the spontaneously breathing pigs. Intrathoracic pressures (ITP) were recorded using a Millar catheter placed in the trachea at the level of the carina. After stabilizing the pigs blood pressure, heart rate, and ventilation rate, intracranial pressures (ICP) and intrathoracic pressures were recorded, with 0 cmH2O inspiratory impedance and then with inspiratory impedances of 5,10,15, and 20 cm H2O. Inspiratory impedance was achieved using an impedance threshold valve (ITV) as described in FIGS. 2–5.

At base, the intracranial pressure was approximately 8/4 mmHg. With increasing amounts of inspiratory impedance, the intracranial pressure was lowered proportionally as shown in FIG. 7. The intracranial pressure was 6/−2 mmHg when the pig breathed through an impedance of 20 cm H2O. These findings were observed in multiple pig studies and were reproducible. Next, the Millar catheter was inserted 3 cm into the pig's brain. The intracranial pressure increased secondary to the trauma associated with the insertion of the probe. The intracranial pressure increased to 25/22 mmHg at the new baseline. Next, the impedance threshold valve was evaluated at different levels of resistance (FIG. 8). Again, there was a decrease in intracranial pressure proportional to the degree of inspiratory impedance. In all examples, the intrathoracic pressure decreased relative to the rest of the body, creating a suction effect that reduced the pressure in the venous blood vessels draining the brain, thereby reducing intracranial pressures.

The invention has now been described in detail for purposes of clarity and understanding. However, it will be appreciated that certain changes and modifications may be practiced within the scope of the appended claims. 

1. A method for decreasing intracranial pressures in a person in need thereof, the method comprising: coupling a valve system to the person's airway, the valve system being configured to at least periodically reduce or prevent respiratory gases from flowing to the person's lungs; repetitively lowering the person's negative intrathoracic pressure using the valve system when coupled to the person's airway to repetitively lower intrathoracic pressures to thereby reduce intracranial pressures; wherein respiratory gases are prevented from entering the lungs through the valve system until a negative intrathoracic pressure in the range from less than about −2 cmH₂O to about −25 cmH₂O is achieved, at which time the valve system permits respiratory gases to flow to the lungs.
 2. A method as in claim 1, wherein the person's negative intrathoracic pressure is repetitively decreased by repeatedly inspiring through the valve system.
 3. A method as in claim 2, wherein the person spontaneously inspires through the valve system to repetitively decrease the intrathoracic pressure.
 4. A method as in claim 2, further comprising artificially causing the person to repeatedly inspire through the valve system.
 5. A method as in claim 4, wherein the person is artificially caused to repeatedly inspire by repeatedly stimulating the phrenic nerve, by manipulating the chest with an iron lung cuirass device, by generating negative pressures within the thorax using a ventilator, or by applying a high frequency ventilator that supplies oscillations at a rate of about 200 to about 2000 per minute.
 6. A method as in claim 1, further comprising fixing an impedance level of the valve system.
 7. A method as in claim 1, further comprising varying an impedance level of the valve system over time.
 8. A method as in claim 7, further comprising measuring at least one physiological parameters of the person, and wherein the impedance level is varied based on the measured parameters.
 9. A method as in claim 8, wherein the parameters are selected from a group consisting of respiratory rate, intrathoracic pressure, intratracheal pressure, blood pressure, heart rate, end tidal CO₂, oxygen level, and intracranial pressure.
 10. A method as in claim 1, wherein the valve system is coupled to a mouthpiece, an endotracheal tube, or a face mask that is coupled to the person's airway.
 11. A method for decreasing intracranial pressures in a person in need thereof, the method comprising: performing a step to actively decrease the person's negative intrathoracic pressure to lower intrathoracic pressures by coupling a valve system to the person's airway to prevent respiratory gasses from entering the lungs using the valve system until a negative intrathoracic pressure in the range from less than −2 cmH₂O to about −25 cmH₂O is achieved, at which time the valve system permits respiratory gases to flow to the lungs thereby reduce intracranial pressures.
 12. A method as in claim 11, wherein the method comprises repetitively decreasing the person's negative intrathoracic pressure.
 13. A method as in claim 11, wherein the step of actively decreasing the person's negative intrathoracic pressure comprises maintaining a vacuum in the thorax relative to the rest of the body.
 14. A method as in claim 11, wherein the step of actively decreasing the person's negative intrathoracic pressure comprises repeatedly stimulating the phrenic nerve, manipulating the chest with an iron lung cuirass device, or generating negative pressures within the thorax using a ventilator. 